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How hard is it to build a bomb? Mr. Biden might want to pay attention.
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• It takes only 8 days to enrich the medical-grade fuel to make 15 kilograms (33 pounds) of 90 percent weapons-grade fuel.
Other expert calculations assume a faster progression through the three stages, but in round figures one can apply two rules of thumb: 10-10-10 for the three stages of material shrinkage listed above; and 11-1-1 for the three time periods: 11 months for commercial reactor fuel, then one month more for medical reactor fuel, then one week for weapons-grade uranium.
To these rules we can add one more number each, to complete the sequences. Adding a final 10 to the 10-10-10 sequence captures the difference between the minimum amount of fuel needed for a crude uranium bomb a terrorist can use (roughly 60 kilograms—the amount used in the Hiroshima bomb), and a conservative estimate of the minimum amount needed for a highly sophisticated plutonium bomb that a first-rank nuclear state can use to optimize its nuclear arsenal (roughly 6 kilograms—some estimate the number even lower). Adding another 1 to the second sequence tells us that once all needed components are in place, it takes about one day to build an operational weapon. (Final assembly of the Nagasaki bomb took one day.)
Enrichment is by far the hardest part of the total work needed to assemble a nuclear device. But a country does not need to intend a weapons program initially. For example, India’s decision to seek nuclear weapons was driven by its high-altitude 1962 border conflict with China, its serial wars with Pakistan, and a series of Chinese tests: of an A-bomb in 1964, an H-bomb in 1966, and a nuclear-capable ballistic missile later that year. By 1966, India was, courtesy of its 10 years of commercial nuclear activity, close to having fuel for a weapon. It detonated its first device in 1974.
As the late historian Roberta Wohlstetter (full disclosure: my aunt) explains in her landmark 1976 study, The Buddha Smiles:
The Indian case…illustrates…that a government can, without overtly proclaiming that it is going to make bombs (and while it says and possibly even means the opposite), undertake a succession of programs that progressively reduce the amount of time needed to make nuclear explosives, when and if it decides on that course.
Thankfully, putting together the vast, industrial-scale infrastructure needed to enrich uranium via these methods is extremely difficult; no terrorist is going to do this in a garage or on the back lawn with presently available methods.
One of the main sources of concern, however, lies in the ability of a state enriching uranium to rapidly assemble a bomb, which—as noted above—need not be a full-scale weapon, but merely a rudimentary device to fit inside a shipping container or a road vehicle. Thus, when the Obama administration uses as the measure of Iran’s weapons status whether it can assemble a modern weapon, the Obama calculus ignores the crude device that can be assembled far more easily and faster—and transferred to Hezbollah.
SO MUCH FOR URANIUM, the fuel of choice for proliferators. But what about plutonium? Plutonium accumulates in the spent fuel collected from nuclear reactors. The U-238 in a nuclear reactor will capture a neutron and, instead of fissioning, become an extremely unstable atom with a combined total of 239 neutrons and protons. In a series of transmutations (changes in chemical composition), this U-239 naturally becomes fissile Pu-239, the most common modern fuel for nuclear weapons.
How a reactor is designed and run determines how readily and conveniently it creates Pu-239. The reactor the Iraqis built in the late 1970s was to run on weapons-grade fuel and was made to maximize plutonium production; Israel understood this perfectly well, and hence destroyed it in 1981—before it was fueled, to avoid scattering radioactive material for miles upon bombing it. Proliferation expert Henry Sokolski writes that a light-water reactor rated at a tenth the size of a commercial plant can be run so as to produce dozens of pounds of plutonium in a year. This is more than enough to fuel several nuclear bombs.
Weapons-grade plutonium makes for a more efficient bomb fuel than weapons-grade uranium, and thus offers more explosive power per pound. The actual amount of plutonium converted into energy inside the core of the Nagasaki bomb was about one gram, or one-third the weight of a penny. Einstein’s E = mc2 equation explains this. The released mass (m) converted into kinetic, thermal, and radiant energy is infinitesimally small—less than a thousandth of the mass that fissioned, as most of what fissioned careens around in search of other nuclei to split. But the “c2” represents, in kilometers per second, the square of the speed of light in a vacuum. Applying this huge multiplier to every atom whose nucleus is split in a detonation yields a vast release of energy (E) in various forms.
A crude uranium bomb is relatively easy to build. The Hiroshima bomb used uranium enriched to 80 percent U-235. Within the bomb, half the uranium was fired—by a miniature version of a World War II warship’s naval gun—into the other half, causing a supercritical mass to form and detonate in microseconds (millionths of a second). The Manhattan Project scientists were so certain a guntrigger design would work that they did not even test it—uranium was in short supply and they needed it to create plutonium for the Trinity test and then the Nagasaki bomb.
But Pu-239 is much harder to make into a nuclear bomb. It must be placed in a special configuration, far more complex than that for a uranium bomb. A plutonium detonation occurs in nanoseconds (billionths of a second), a thousand times faster than a uranium detonation. To make sure as much of the plutonium as possible fissioned, the Trinity and Nagasaki bombs were “implosion” devices. A complicated arrangement of 32 symmetrically spaced conventional explosives surrounded those bombs’ plutonium cores. Thirty-two lenses converted the shock waves from convex to concave, to compress the plutonium core extremely rapidly and evenly. A timing discrepancy among the implosion lenses of 10 microseconds—10 one-millionths of a second—reduces symmetry and can create a dud; a timing discrepancy of just one microsecond is enough to create a partial dud. In essence, plutonium bombs require superspeed, supersymmetry, and supersmall compression.
For a nuclear weapons state seeking to arm missiles, plutonium is the fuel of choice, because it provides more yield per pound, and thus is more suitable for small warheads. It is very unlikely that terrorists would be able to build a plutonium fission device on their own, due to the extreme sophistication involved.
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